This invention relates to a method of manufacturing high quality coated paper for printing and, more particularly, to a method of manufacturing a high quality coated paper for printing, which has excellent smoothness, gloss and printability, by using latexes of particular heterogeneous polymer particles as an ingredient of the coating composition and high temperature calenders.
Recently, there has been an increasing demand for improving the gloss, visual gloss, smoothness, ink receptivity and ink gloss of the coated surface of coated paper for printing to meet a trend for visualization of print, color printing and high quality printing. In fact, various processes for manufacturing coated paper for printing have been proposed.
For example, a multi-layer coating process using a blade coater has been proposed, and in a finishing step there has been proposed the use of a high temperature calender maintained at 100.degree. C. or more (as disclosed in, for example, U.S. Pat. No. 4,241,143 and Japanese Patent Publication No. 49-21252). Moreover, it is known to use, as pigments of a coating composition, fine particles such as satin white, ultra-fine ground calcium carbonate, fine cubic precipitated calcium carbonate, delaminated kaolin, calcined kaolin, fine talc and plastic pigments or to apply a concentrated coating composition having a solid content of 65 wt % or more to a base paper. However, according to these prior art methods, it is impossible to obtain a high quality coated paper which has excellent gloss, visual gloss and smoothness and further has excellent printability, including ink receptivity and ink gloss.
A method for manufacturing coated print paper having high gloss is well known in the art. For example, one method comprises the steps of coating an aqueous coating composition mainly composed of a pigment and an adhesive on base paper, drying the coated layer on the paper and then finishing it with a gloss calender at a high temperature. There is used kaolin, satin white, calcium carbonate, plastic pigment, etc. as the pigment component of the composition and a polymer latex with a glass transition temperature (hereinafter referred to as Tg) of 38.degree. C. or more is used as the adhesive component of the composition. The coating layer is dried at a temperature lower than the Tg of the polymer latex and then finished at a temperature higher than the Tg (as disclosed in, for example, U.S. Pat. Nos. 4,265,969 and 4,241,143). However, with a coating composition containing a polymer latex and a plastic pigment, the content of The adhesive should be greatly increased as compared with the case of using kaolin or the like because the particle diameter of the plastic pigments is usually as small as 0.6 micron. More specifically, the composition should contain at least 12 wt % of plastic pigment in order to obtain improvements in gloss or visual gloss by the use of the plastic pigment. In this case, it is necessary to use an adhesive in an amount of 20 wt % or more on the basis of the solid content of the coating composition. This means that the sum of the amounts of the plastic pigment and adhesive exceeds 32 wt % of the solid content of the coating composition, that is, the proportion of expensive materials is very high. In addition, a coating composition using a polymer latex as an adhesive component spoils the operation of the coater due to the fact that a coating layer containing a polymer latex should be dried at a temperature which does not result in the forming of a polymer latex film. For the above reasons, the coating composition as mentioned above is hardly used in practice in the manufacture of coated paper.
For the purpose of improving the gloss of coated paper, there has been proposed a process which comprises the steps of applying to a base paper a coating composition containing a copolymer latex of heterogeneous polymer particles composed of a core polymer and a shell polymer, said core polymer having a Tg of 60.degree. C. or more and not being film-forming under the drying conditions employed in the manufacture of the coated paper, said shell polymer being film-forming under said drying conditions and being able to form a continuous film at a normal temperature; drying the coating layer of the composition on the base paper; and then subjecting the dried coating layer to finishing with a supercalender (as disclosed in Japanese Patent Laid-Open No. 54-151606 and U.S. Pat. Nos. 4,381,365 and 4,613,633). However, where the core/shell type polymer latex as mentioned above is added to a coating composition used in coated paper production employing a finishing step with a high temperature calender at more than 100.degree. C., as is intended by the present invention, the core polymer softens and unites with the shell polymer in the finishing step. Therefore, the use of the aforesaid heterogeneous polymer latex of the core/shell type does not produce a good result as compared with the use of a homogeneous polymer latex and it is impossible for it to satisfactorily improve the smoothness and gloss of the coated paper.